Stabilization of metal treating solutions

ABSTRACT

A NOVEL AND IMPROVED METAL TREATMENT SOLUTION AND PROCESS FOR THE TREATMENT OF METALS THEREWITH. MORE PARTICULARLY, THIS INVENTION RELATES TO AN ANHYDROUS CHROMIC ACID METAL TREATING SOLUTION AND A PROCESS FOR TREATING METAL ARTICLES TO IMPART THERETO A HIGHER RESISTANCE TO CORROSION AND AN IMPROVED BONDING OF PAINTS LACQUERS AND THE LIKE THERETO. CERTAIN STABILIZERS WHICH INCLUDE ZINC CADMIUM, MERCURY AND SALTS THEREOF, EFFECTIVELY STABILIZE THE METAL TREATING SOLUTIONS TO PROVIDE IMPROVED SOLUTIONS AND PROCESSES FOR THE TREATMENT OF METALS.

United States Patent O US. Cl. 10614 11 Claims ABSTRACT OF THE DISCLOSURE A novel and improved metal treatment solution and process for the treatment of metals therewith. More particularly, this invention relates to an anhydrous chromic acid metal treating solution and a process for treating metal articles to impart thereto a higher resistance to corrosion and an improved bonding of paints, lacquers and the like thereto. Certain stabilizers which include zinc cadmium, mercury and salts thereof, effectively stabilize the metal treating solutions to provide improved solutions and. processes for the treatment of metals.

BACKGROUND OF THE INVENTION The treatment of metal surfaces with chromic acid formulations in aqueous solutions is known in the art. Such processes have not been satisfactory since they involve numerous steps and have not imparted a satisfactory corrosion resistance nor satisfactory paint adhesion to the metal surface. Moreover, aqueous chromic acid solution treatments require a drying operation prior to painting.

Nonaqueous metal treating systems, as described in US. Pat. 3,285,788, have been in recent use. These are commonly referred to as Trichromatizing solutions. Trichromatizing solutions for placing a chromate coating on a metal substrate comprise a halogenated hydrocarbon solvent, an alcohol (preferably tertiary) and chromic acid. Typically, the solvent is trichloroethylene and the alcohol is t-butyl alcohol. Since chromic acid is insoluble in trichloroethylene, the t-butyl alcohol is added to form a chromate ester which is soluble in trichloroethylene. Generally, it is not possible to form stable chromate esters of primary and secondary alcohols due to the powerful oxidizing nature of chromic acid. In the presence of tertiary alcohols, it is possible to form the esters. However, even these esters are not highly stable especially above room temperature. They are susceptible to hydrolysis by traces of water and undergo internal oxidation-reduction reaction to form lower valence, insoluble chromium compounds. These degradation reactions are deleterious to the effective operation of the process. They rapidly reduce the concentration of the soluble Cr+ reaction species necessary for coating the substrate and cause an accumulation of sludge.

It is, therefore, apparent that metal treating solutions which impart improved corrosion resistance and paint adhesion to metals are needed.- Additionally, improved processes for the treatment of metals are likewise in demand.

SUMMARY OF THE INVENTION This invention relates to a metal treating solution comprising a chlorinated hydrocarbon, chromic acid, tertiary ice alcohol and a stabilizing agent selected from the group consisting of zinc, cadmium, mercury, and the nitrates, chlorides, borates, oxides and organic salts thereof.

In addition, the invention involves a process of treating metal to increase its resistance to corrosion comprising applying to the metal a solution which contains a chlorinated hydrocarbon, chromic acid, tertiary alcohol and a stabilizing agent selected from the group consisting of zinc, cadmium, mercury, and the nitrates, chlorides, borates, oxides and organic salts thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred metal treating solution comprises a chlorinated hydrocarbon containing 0.001% up to the saturation point of chromic acid, 0.001% to 30% of a tertiaryalcohol which is soluble in the chlorinated hydrocarbon, and a stabilizing amount of a stabilizing agent selected from the group consisting of zinc nitrate, zinc chloride, zinc borate, zinc metal, zinc acetyl acetonate, cadmium metal, cadmium oxide, cadmium chloride, cadmium borate and mercury chloride.

This invention concerns the use of zinc and cadmium, and mercury additives to effectively maintain the soluble Cr+ esters at a high level in the trichromatizing bath. Research work has demonstrated that the bath stability is affected by many factors including the initial concentrations of alcohol and chromic acid. Using a given set of operating conditions, it has been discovered that certain salts of zinc, cadmium and mercury, as well as the metals themselves, decrease the degradation rate of the soluble Cr+ coating esters by as much as a factor of 5.

Generally, the stabilizing agents are added to the metal treating solutions in a stabilizing amount (i.e., in an amount which will effectively stabilize the solution). Typically, the numerical ranges are between about 0.001% and about 5%; a preferred range is 0.01-1%. The stabilizing agents include zinc nitrate, zinc chloride, zinc borate, zinc metal, zinc acetyl acetonate, zinc stearate, zinc oleate, zinc naphthenate, cadmium metal, cadmium oxide, cadmium chloride, cadminum borate, mercury chloride, mercury nitrate and mixtures thereof. It has also been discovered that the organic salts of zinc, cadmium and mercury prevent the build up of sludge; the saturated organic salts of these metals are preferred in preventing sludge buildup.

The metals treated by the process of this invention include aluminum, aluminum alloy, iron, steel or zinc structures. In particular, iron, galvanized iron or aluminum strip, may be treated by applying to such structure, for example by immersion, painting, spraying or the like, a coating of the treating solution, and then, optionally, subjecting the coated surface to a fixation medium including ultraviolet light, heat or a reducing gas; a direct flame is a preferred treatment.

By the term chromic acid" as used herein is meant chromic anhydride, chromium trioxide, CrCO or the hypothetical substance H CrO Reference to quantities of chromic acid are expressed in terms of the equivalent anhydride (CrO By the term aluminum alloy is meant one of the well-known series of alloys containing not less than about aluminum and minor amounts of alloying ingredients such as Cu, Mn, Mg, Si, Zn or Cr. For example, Alloy 2021 is composed of 4.5% Cu, 0.6% Mn, 1.5%

Mg and the balance Al. Alloy 6061 is composed of 0.25% Cu, 0.6% Si, 1% Mn, 0.25% Cr and the balance Al.

' Alloy 7075 is composed of 1.6% Cu. 2.5% Mg. 5.6%

Zn, 0.3% Cr and the balance Al. Alloy 3003 is composed of 1.2% Mn and the balance Al.

By the terms aluminum, aluminum alloy, iron, steel, or zinc structure is meant structures composed entirely of these metals or metal substrates containing coatings of aluminum, aluminum alloy, iron, steel or zinc.

In the preparation of the anhydrous chromic acid solution of this invention, the base of the solution is comprised of a chlorinated hydrocarbon solvent. As specific examples of such solvents suitable for use in the preparation of the chromic acid solutions are trichloroethylene, perchloroethylene, carbon tetrachloride, chloroform, methylchloroform, methylene chloride and mixtures thereof. Depending upon the mode of operation, the chromic acid is added to the solvent in an amount of between about 0.001% by weight up to the saturation point thereof or even in excess of the saturation point. The chromic acid is dissolved in the solvent by the presence in the solution of tertiary butanol, or other tertiary alcohol, soluble in the solvent, containing 4 to 20 carbon atoms, as a solubilizing agent. The tertiary alcohol should be present in an amount of at least 0.001% by weight and it may be present in an amount up to 30% or higher, the only limit being the objectionable flammability of the solution if present in too great an amount. The greater the quantity of tertiary alcohol present, the greater the quantity of CrO that will dissolve in the solution since CrO reacts with the alcohol to form chromate esters.

Of the tertiary alcohols, tertiary butanol appears to be outstanding as a solublizing agent being operative for extended periods of time. Other tertiary alcohols, soluble in chlorinated hydrocarbon solvents, that may be used include tertiary amyl alcohol; 1,1-dimethyl 2,2-dichloropropanol; 1,1,2,2-tertamethylpropanol and triphenyl carbinol.

The stabilizing agents have a pronounced effect on the solutions. For example, chromic acid in solution in trichloroethylene containing between about 0.001% up to its saturation point at reflux of zinc nitrate will be 50% reduced after about 72 hours with the solvent at reflux temperature. A similar chromic acid solution containing no zinc nitrate will lose 50% of the chromic acid in only 15 hours.

The preferred process of treating the metals is carried out by the following steps:

First, it is preferred, although not essential, to brush the metal with a wire brush or sandblast or otherwise abrade the surface thereof.

The metal (e.g., aluminum, alloy, iron, steel or zinc) is then degreased by a conventional degreasing process with trichloroethylene or perchloroethylene, i.e., by immersing the same in trichloroethylene or perchloroethylene liquid or vapor, or both, until the metal is clean. This, however, is purely a cleaning step and if the metal is clean, can be omitted.

The metal is then immersed in the anhydrous chromic acid treating solution. Although the treating solution may be at any temperature between room temperature and the boiling point of the solvent, it is prefered for economical reasons to operate the treating step at reflux temperature of the solution. The solution is heated to its boiling point and the vapors are condensed by cooling coils surrounding the top inner surface of the treating vessel. Trichloroethylene, methylene, chloride, perchloroethylene and mixtures thereof are therefore eminently suitable as the base solvent in the solution.

The metal structure is contacted with the treating solution, for example, between about 0.1 second and 5 minutes. Under some circumstances it may be desirable to coat the metal with the treating solution repeatedly. When the metal is removed from an immersion in the treating solution operating under reflux conditions, it

will necessarily pass through the vapor zone of the solvent. In order to prevent washing off the coating, it should be passed through the vapor zone quickly to prevent undue condensation of the vapor on the metal, or the vapor zone may be reduced to minimize condensation of vapor on the metal.

The chromium-containing coating formed on the metal is a uniform thin powdery layer and does not at this point withstand extensive handling. The coated material is therefore subjected to a treatment with ultraviolet light, heat or a reducing gas. Any source of ultraviolet light, i.e., emitting rays in the range of 1800 A. to 3900 A. can be used, for example, an ultraviolet light-emitting sun lamp. The coated surface should remain in the presence of the ultraviolet light for a period of l to 300 seconds depending upon the source of the ultraviolet light and the spacing of the light source from the surface. Thirty seconds exposure at a spacing of 1 inch with a Hanovia type SH bulb sold by Englehard Hanovia, Inc., Newark, N.J., will produce the desired setting of the coating to permit handling.

The chromium-containing coating may be set by subjecting the coated structure to heat, for example in an oven having a temperature between 100 and 600 C. At a temperature of 100 C. the coated structure may have to be heated for a period as long as 15 to 20 minutes to obtain the necessary set, whereas at an oven temperature of 600 C. the CrO coating will set in 5 to 10 seconds. The heating of the panels may be carried out in any desired manner such as by direct flame, oven, electric heat lamp or the like.

The CrO coating may also be set by treating the same with a reducing gas for a period of 5 seconds to 10 minutes depending upon the particular reducing gas used and the temperature thereof. Gases such as hydrogen, hydrogen sulfide, formaldehyde, sulfur dioxide and hydrogen peroxide may be used for this purpose. The exposure of the chromium oxide coated surface to ultraviolet light, heat or a reducing gas as above described functions to fix the chromium-containing film on the metal surface causing it to become substantially integral with the surface.

The chromate treatment of this invention may be used to great advantage as an after treatment on aluminum, aluminum alloys, iron, steel or zinc which have first been treated with a phosphatizing composition. For example, a number of galvanized iron panels were phosphatized by submersing them in a solution of trichloroethylene containing 5% amyl alcohol and 0.2% phosphoric acid having a temperature of 87 C. for a period of 60 seconds.

In order to illustrate the invention and demonstrate the effects of selective stabilizing agents in chromatizing solutions, the following procedure was performed. Metal treating solutions containing, in Weight percent, 5% tertiary butyl alcohol, 0.35% chromic acid and trichloroethylene were formulated in a 500 cc. Erlenmeyer flask. To this flask containing the chromatizing solution, various amounts of stabilizing agents were added. The flask was then connected to a reflux condenser to avoid solution losses and brought to reflux at about 82 C. At the same time a control solution without any stabilizing agent was refluxed. At preselected time intervals, 10 cc. portions of the solution were withdrawn for analysis. The chromic acid content of the bath was analyzed using a standard iodine-thiosulfate titration as described in Quantitative Analysis, Pierce and Haenisch, 3rd edition, J. Wiley and Sons, page 237.

The results, which were calculated as percent CrO were then plotted as the' log percent'CrO remaining in the solution vs. thetime in hours. Generally, such a plot gave 'a straight line. From these plots, it was possible to original value. The results are re- TABLE 1 Amount, Time, hours 1 weight Stabilizing agents percent t50% t75% None 15 33 Zl'l(NO3)z-6Hz0 0.282 72 100 0. 578 55 80 0. 062 40 70 0. 102 40 80 0. 367 42 68 0. 260 39 50 0. 124 31 60 Zn acetyl acetonate 0. 158 29 53 Zn stearote 0. 607 21 47 Zn oleate 0. 300 19 42 Zn naphthenate- 0. 695 17 39 1 For traction of CrO lost.

The following examples illustrate the comparative paint adhesion of steel panels which were treated with a stabilized chromatizing solution of this invention. The

paint was an alkyd baking enamel having approximately the following formulation:

Percent Pigment 31.2 Urea-formaldehyde-butyl alcohol resin (60% solids) 24.35 55% castor oil modified glyceryl phthalate (60% solids) 30.3 Hydrocarbon solvent 12.25 Candelilla wax dispersion (15% solids) 0.2 Tertiary-butyl alcohol 1.7

EXAMPLE 1 Another steel panel was degreased with trichloroethylene and hten dipped in a chromatizing bath to produce a coating weight of 9.6 mg. CrO /ft. The chromatizing solution contained 0.68% CrO 5% tertiary butanol, 0.125% Zn(NO -6H O and the remainder trichloroethylene. Thereafter, the panel was subjected to a direct flame to aflix the chromate coating.

This panel was then painted and tested as described in Example 1. After 574 hours in the salt spray, paint layback was 3 mm. while no paint cracking or delamination was observed.

It is apparent that the metal treating solutions of this invention are more stable and have longer bath life than the previous solutions. Consequently, a chromatizing process can be operated more efiiciently and effectively for longer periods of time. Additionally, the use of organic salts alleviates the sludge buildup problem associated with prior chromatizing baths.

Throughout the specification and claims, any reference to parts, proportions and percentages refers to parts, proportions and percentages by weight unless otherwise specified.

I claim:

1. A metal treating solution comprising a chlorinated hydrocarbon containing 0.001% by weight up to the saturation point of chromic acid, 0.001 to 30% by weight of a tertiary alcohol containing 4 to 20 carbon atoms and a stabilizing amount of a material selected from the group consisting of zinc, cadmium and mercury metals the nitrates, chlorides, and borates of said metals, the oxides of cadmium and mercury, zinc acetyl acetonate, zinc stearate, zinc oleate, zinc naphthenate and mixtures thereof.

2. A metal treating solution in accordance with claim 1 wherein the stabilizing agent is zinc nitrate.

3. A metal treating solution in accordance with claim 1 wherein alcohol is tertiary butyl alcohol.

4. A metal treating solution in accordance with claim 1 wherein the amount of stabilizing agent is within the range of 0.001-5%.

5. A metal treating solution in accordance with claim 1 wherein the amount of stabilizing agent is within the range of 0.01-1%.

6. A process of treating metal to increase its resistance to corrosion comprising applying to the metal a solution comprising a chlorinated hydrocarbon containing 0.001% by weight up to the saturation point of chromic acid, 0.001 to 30% by weight of a tertiary alcohol containing 4 to 20 carbon atoms and a stabilizing amount of a material selected from the group consisting of zinc, cadmium and mercury metals the nitrates, chlorides, and borates of said metals, the oxides of cadmium and mercury, zinc acetyl acetonate, zinc stearate, zinc oleate, zinc naphthenate and mixtures thereof.

7. A process in accordance with claim 6 wherein the chlorinated hydrocarbon is trichloroethylene.

8. A process in accordance with claim 6 wherein the chlorinated hydrocarbon is perchloroethylene.

9. The process in accordance with claim 6 wherein said solution is maintained at reflux temperature.

10. A process in accordance with claim 6 wherein the treated metal is subjected to a direct flame for a period of 1 second to 10 minutes.

11. A metal structure having been treated in accordance with the process of claim 6.

References Cited UNITED STATES PATENTS 11/1966 Svadlenak l48--6.2 4/1969 Svadlenak l486.2

JULIUS FROME, Primary Examiner L. B. HAYES, Assistant Examiner 

